Birch bark is a low-value waste product in the forest industry today. Ekman, R., Holzforschung, (1983) 37, 205. Approximately 230,000 tons of birch bark are generated per year. For example, a single paper mill can generate 70 tons of birch bark per day. Thus, vast quantities of birch bark and its chemical components are available.
Birch bark is a potential source of a variety of organic chemicals. Several triterpenoids have been identified in birch bark extracts. For example, lupeol, betulin, betulinic aldehyde, betulinic acid, methyl betulinate, lupenone, betulonic aldehyde, betulonic acid, β-amyrin, erythrodiol, oleanolic aldehyde, oleanolic acid, methyl leanolate and acetyl oleanolic acid are all present in the outer bark of Betula verrucosa. Eckerman, C., (1985) Paperi ja Puu, No. 3, 100. In addition, several suberinic acids, ω-hydroxy fatty acids, have also been identified in the bark of Betula verrucosa. Ekman, R., (1983), Holzforschung, 37, 205.
The chemical constituents of birch bark are useful in pharmaceutical and industrial applications. For example, U.S. Pat. No. 5,750,578 discloses that betulin possesses antiviral properties and is useful to treat herpesvirus. Betulin also possesses antifeedant activity against boll weevils, and anti-inflammatory activity (Miles, D. H., (1994), J. Agric. Food. Chem., 42, 1561-1562, and Recio, (1995), M., Planta Med., 61, 9-12. In addition, betulin showed cough suppressant and expectorant effects. Jinuhua, W., Zhongguo Yaoxue Zazhi, (1994), 29(5), 268-71. Betulin is also a useful starting material for preparing allobetulin and derivatives thereof, which possess useful pharmacological properties.
Betulin can be converted to betulinic acid, which is useful as a therapeutic agent. For example, Pisha, E. et al., (1995), J. M. Nature Medicine, 1, 1046-1051, discloses that betulinic acid has antitumor activity against human melanoma, e.g., MEL-1, MEL-2 and MEL-4. In addition, Fujioka, T. et al., (1994), J. Nat. Prod., 57, 243-247 discloses that betulinic acid has anti-HIV activity in H9 lymphocytic cells.
Betulin has been extracted from the bark of white-barked birches in amounts up to 30%, based on the dry weight of the bark. Elkman, R., (1983), Holzforsch, 37, 205; Ohara, S., et al., (1986), Mokuza Gakkaishi, 32, 266. Additionally, betulin has been isolated from outer birch bark waste of Betula verrucosa by liquid extraction employing boiling organic solvents and subsequent recrystallization. Eckerman, C., (1985), Paperi ja Puu, No. 3, 100. While current processes afford betulin, these processes suffer from drawbacks. For example, the use of an organic solvent alone in the extraction of betulin may not result in the extraction of betulin that is found in a bound state in the birch bark, thus yielding less betulin than is actually present in the raw bark material.
Russian Patent Nos. RU2175326 (publication date 27 Oct. 2001) and RU2192879 (publication date 20 Nov. 2002), discloses methods of recovering betulin, and derivatives thereof, from birch bark. The methods disclosed in Russian Patent No. RU2192879 include birch bark milling, separation of birch bark fibers, solvent extraction of birch bark, separation of a solution from extracted birch bark, and removal of solvent from the extract. The extraction is carried out with toluene at temperatures of 90° C.-110° C. for 1.5-3.0 hours, and the solution is filtered at a temperature of 40° C.-50° C. The solution of betulin in toluene is cooled for 6-10 hours to a temperature of 15° C.-5° C. for crystallization of betulin.
Published U.S. Patent Application US 2003/0153776 A1 (“the '776 patent application) describes a process for obtaining betulin from birch bark. The process comprises extracting birch bark with a, water-immiscible solvent, and washing this extract with a dilute aqueous base, to provide betulin. Only a 4 wt. % yield of betulin is obtained by the process of the '776 patent application, and no other triterpenoids (e.g., lupeol, betulinic acid, or a combination thereof) are stated to be recovered. The use of charcoal, which is also believed to decrease the yield of betulin, is employed. The methods described in the '776 patent application are not able to effectively extract other naturally occurring triterpenoid derivatives such as betulin-3-caffeate, betulinic acid, or lupeol, or other organic materials such as esters of fatty acids, fatty acids, polyphenols, or tannins from the birch bark. Thus, the yields and purities of betulin resulting from the processes described in the '776 patent application have a need to be improved upon.
Another drawback to several of the currently used extraction processes employed to isolate betulin and other components in birch bark is that particularly hazardous organic solvents such as methylene chloride and chloroform are employed, which are toxic, carcinogenic, costly to dispose of, and pose a threat to the environment.
Thus, there exists a need for processes for recovery of betulin and other valuable natural organic compounds from birch bark that give higher yields of purer materials, use safer solvents, and minimize environmental hazards.
At least some of the inventors herein have previously disclosed and claimed other processes for isolation of natural products from birch bark. For example, see U.S. Pat. Nos. 6,392,070, 6,634,575, 6,768,016, and 6,815,553, and published application US 20050158414, all by Krasutsky, et al., which are incorporated herein by reference.
Suberins are another major component of birch bark. Suberins are a class of waxy water-insoluble polyester materials that are disposed in the birch bark. Kola, P. E. et al., (1981), Ann. Rev. Plant. Physiol., 32, 539-67. Suberins are polyesters of hydroxylated fatty acids and polymeric polyphenolic constituents. In situ suberin is a macromolecular network insoluble in all solvents. The suberins of birch bark are typically polyesters of ω-hydroxy fatty acids with dicarboxylic fatty acids. These polyesters may further be hydroxylated or epoxidized. Ekman, Holzforschung, (1983), 37, 205-211.
Suberins possess several industrial applications. See, e.g., Taylor and Francis, (1998), Forests Products Biotechnology, A. Bruce and J. W. Palfreyman (editors), 167, 179-181; Peter E. Laks and Peggy A. McKaig, (1988), Flavonoid Biocides: Wood Preservatives Based on Condensed Tannins, Horzforschung, 42, 299-306; Etherington & Roberts Dictionary, definition of birch(bark), http://sul-server-2.stanford.edu/don/dt/dt0328.html, 1, Jun. 23, 1999; P. E. Kolattukudy, (1981), Structure, Biosynthesis, and Biodegradation of Cutin and Suberin, Ann. Rev. Plant Physiol., 32, 539-67, and N. Cordeiro, M. N. Belgasem, A. J. D. Silvestre, C. Pascol Neto, A. Gandini, (1998), “Cork Suberin as a new source of chemicals,” Int. Journal of Biological Materials, 22, 71080. Suberin is useful as a dispersant in many industrial applications (e.g., carbon black slurries, clay products, dyes, cement, oil drilling muds, and asphalt emulsifiers). Suberin is also useful in binders for animal pellets, conditioners for boiling water, anti-oxidants and additives to lead-storage battery plate expanders. McGraw-Hill Concise Encyclopedia of Science & Technology, 4th Ed., 1998.
Several fatty carboxylic acid derivatives, known collectively as suberinic acids, may be derived from saponification of suberin and other natural polyesters found in birch bark. The compound 9,10-Epoxy-18-hydroxyoctadecanoic acid is one such suberinic acid. Specifically, 9,10-Epoxy-18-hydroxyoctadecanoic acid has been found to protect leaves of a highly susceptible barley cultivar against fungal pathogen Erysiphe graminis f.sp. hordei. Sweitzer, P., et al., (1996), “Induction of Resistance in Barley Against Erysiphe graminis by Free Cutin Monomers,” Physiol. Mol. Plant Pathol, 49(2), 103-120. This fatty acid derivative is of an usual type in nature in that it bears a hydroxyl group on the co-carbon, that is, the carbon at the distal end of the chain from the carboxylate moiety. Functionalization of this position of a fatty acid is unusual in nature, and is also difficult to achieve synthetically. Therefore, such compounds represent valuable intermediates for preparation of organic compounds.
Another suberinic acid which may be recovered from birch bark is 9,10,18-trihydroxyoctadecanoic acid. This compound is a useful precursor for the synthesis of ambrettolide. Ambrettolide (cis-hexadec-7-enolide), which is also found naturally occurring in the vegetable oil of ambrette seeds, is used as a musk fragrance in perfumes. The synthesis of ambrettolide may accomplished from 9,10,18-trihydroxyoctadecanoic acid via a high-yielding multi-step synthesis. Seoane, E., (1982), J. Chem. Soc. Perkin Trans., 1837-1839.
A need therefore exists for environmentally safer, more cost-efficient methods to obtain commercial quantities of betulin, lupeol, betulinic acid, 9,10-epoxy-18-hydroxyoctadecanoic acid, and 9,10,18-trihydroxyoctadecanoic acid from birch bark.